1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program such as a wireless LAN (Local Area Network) or PAN (Personal Area Network) for communicating between a plurality of wireless stations. In particular, the invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for allowing communication apparatuses to operate in an autonomous distributed manner without the relationship between a controlling station and a controlled station.
More specifically, the invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program by which at least a part of the communication apparatuses notify a beacon containing information concerning a network so that adjacent communication apparatuses identify each other to operate an autonomous distributed network. In particular, the invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for forming a network while ensuring security and for exchanging necessary data, avoiding interference, between communication apparatuses of adjacent network groups configured by notifying respective beacons.
2. Description of the Related Art
Wireless networks draw attention as systems that free users from cable wiring in traditional wire communication schemes. Canonical standards for the wireless networks include IEEE (the Institute of Electrical and Electronics Engineers) 802.11.
According to a general method of constructing a local area network using a wireless technology, there is provided one apparatus called an “access point” or a “coordinator” functioning as a control station inside the area. The network is formed under overall control of the control station. For this kind of wireless network there is broadly used an access control method based on band reservations, in which communication apparatuses synchronize with each other through the mediation of the access point. In other words, when a given communication apparatus transmits information, a band needed for the information transmission is firstly reserved through the access point so that the communication apparatus can use a transmission path without conflict with the information transmission by the other communication apparatuses.
In the case of performing asynchronous communication between transmitting and receiving communication apparatuses in a wireless communication system having an access point, wireless communication always needs to be routed through the access point, thereby causing a problem of a significant decrease in utilization efficiency of a transmission path. For this reason, as another method of constructing a wireless network, there is proposed a so called “ad-hoc communication” for directly performing asynchronous communication between communication stations operating autonomously. Particularly, in a small-scale wireless network composed of a relatively small number of clients that locate close to each other, it is considered that such ad-hoc communication is suitable because any terminal can directly perform asynchronous wireless communication without using a particular access point.
An ad-hoc wireless communication system has no central control station. Accordingly, this system is suited for constructing a home network composed of household electric appliances. The ad-hoc network has several features. For example, if one apparatus fails or is powered off, the routing is automatically changed, making the network strong against crash. Since a packet is hopped more than once between mobile stations, data can be transferred to a relatively distant destination with a high data rate maintained. There are known various development examples about the ad-hoc system (e.g., see non-patent document 1).
For example, communication apparatuses that perform autonomous network operations transmit the respective beacons at every predetermined period, and a communication apparatus that intends a new entry receives beacon signals concentratedly by a scan operation over the predetermined period and recognizes a communication apparatus that exists within the wireless coverage. According to this method, since adjacent communication apparatuses can set beacon transmission timings in a random fashion, it is possible to suitably configure an autonomous distributed ad-hoc network by communication apparatuses that operate on an equal basis (e.g., see patent document 1).
If communication apparatuses set each individual beacon signal timing in a predetermined period, the beacon transmission positions of communication apparatuses in the same network group are distributed in arbitrary positions, thereby making it difficult to grasp the states of adjacent communication apparatuses. This is because a scan operation over a whole predetermined period is required to detect a beacon transmission from a new communication apparatus. While a scan operation over a predetermined period makes it possible to grasp the states of adjacent communication apparatuses, frequent scan operations increase power consumption. On the contrary, the longer the interval between scan operations, the less information concerning the adjacent state is obtained.
For this reason, in the latest specification concerning a multiband OFDM system (MBOA-MAC), a study is being made on a method by which the communication apparatuses of the same network group define a specified superframe period, specify a beacon period shared by the communication apparatuses using a part of the superframe period, transmit and receive beacons among themselves in this beacon period, and thereby manage the network (e.g., see non-patent document 2). According to this method, the beacon transmission timings of the communication apparatuses exist integratedly at a specified time in a superframe period so that the communication apparatuses can easily grasp the states of adjacent communication apparatuses.
In the case where a communication apparatus that intends a new entry detects a plurality of beacon periods, merging the beacon periods into one of them enables the utilization of the above feature. In MBOA-MAC, a study is being made on a method for performing management by merging into one beacon period to form the same beacon period in the case of detecting a communication apparatus of a different beacon period. For example, a user of a beacon slot located in a rearward position utilizes a forward beacons lot of no beacon transmission, thereby enabling a beacon period to be a minimum necessary period. Alternatively, communication apparatuses specified by users constitute a network group having one beacon period and thereby can operate independently of communication apparatuses in another network group of a different beacon period.
In general, a network is formed among apparatuses specified by users that have gone through predetermined authentication processes, as a method for forming a network group among adjacent communication apparatuses.
In the case where there exists an access point in a network, there is employed a method of link management for making a one-to-one connection between the access point and a terminal of communication permission. For example, in a wireless LAN device in conformity with IEEE802.11, the same SSID (Service Set Identifier) set in a communication apparatus to be an access point is also registered in a communication apparatus to be a terminal by a user, thus performing an authentication process (e.g., see patent document 2).
On the other hand, in the case of an ad-hoc system having no access point disposed, it is necessary to exchange data between communication apparatuses specified by users that have gone through predetermined authentication processes. In an ad-hoc network, while communication apparatuses can constitute a network without being aware of a network group, there is a possible method of making settings such that data communication is valid only among communication apparatuses of the network.
In the network configuration method for setting a beacon period, in the case of performing data communication among communication apparatuses that have already formed authentication relationships, it becomes easier to establish one beacon period by exchanging authentication information beforehand. Further, it is possible to enter a network specified by a user based on information obtained by a prior authentication process, thereby reducing security problems in the network.
The present inventors consider that in an autonomous distributed network, it is necessary to equally manage the existence of communication apparatuses of another network group that have not gone through authentication processes as well as communication apparatuses of their network group. However, in a communication environment where ad-hoc networks are adjacent to each other in which communication apparatuses set each individual beacon period and operate independently and equally, if a communication apparatus communicates with any communication apparatus in a random fashion and on an equal basis without authentication relationships with other network groups, the risk of occurrence of a security problem becomes high.
In a configuration where a scan operation is activated by transmitting a scan request to another communication apparatus, it is difficult to eliminate a risk that a malicious communication apparatus transmits more scan requests than are necessary.
It is difficult to detect a communication apparatus operating with a different beacon period, using a method by which communication apparatuses transmit and receive beacons between themselves in a common beacon period specified by the communication apparatuses. A method being studied as the current MBOA-MAC Layer specification does not permit entry into a different beacon period, thereby preventing reservation information concerning a communication apparatus of a different beacon period from being propagated.
For this reason, if communication apparatuses operating with different beacon periods spatially overlap each other in wireless coverage, there is a problem that a conflict arises between timings of reservation communications so that the throughput of data communication decreases.
Although there is a possible method of operation merging different beacon periods into one in the case of detecting a communication apparatus operating with a different beacon period (described above), it is assumed that it is difficult to merge different beacon periods into one in a communication environment changing drastically. For example, in a commuter train or a crowd, innumerable mobile networks always overlap one another in wireless coverage and the merger of beacon periods occurs frequently, thereby making it rather difficult to merge different beacon periods into one.
In the method of operation merging different beacon periods of communication apparatuses into one, a communication apparatus communicates with any communication apparatus in a random fashion and on an equal basis, thereby increasing the risk of occurrence of a security problem.
In the case of using a vacant forward beacon slot during a beacon period (described above), separating one beacon which different beacon periods have been merged into brings about a plurality of different networks having the same beacon start position, that is, networks that operate with timings forwarded in the same way. In such a case, recombining networks in which vacant forward beacons are used during the same beacon period causes a conflict of beacon slots with certainty because only forwarder beacon slots are used.
In summary, in a wireless communication system that shares one beacon period in the same network group, in the case where a communication apparatus detects a plurality of beacon periods after power-on, the communication apparatus activates processing for merging the beacon periods of two or more network groups operating independently into one beacon period. Accordingly, reconfiguration is required for an existing network that has been operating with stability, thereby making it difficult to perform stable network operation.
[Patent document 1] Japanese Patent Application Laid-Open No. 2004-228926, paragraph number 0064, FIG. 4
[Patent document 2] Japanese Patent Application Laid-Open No. 2004-187001, paragraph number 0005
[Non-patent document 1] “Ad Hoc Mobile Wireless Network” by C. K. Tho (Prentice Hall PTR)
[Non-patent document 2] MBOA-MAC Spec. v090